[lldb] [Process/NetBSD] Copy the recent improvements from FreeBSD

Copy the recent improvements from the FreeBSDRemote plugin, notably:

- moving event reporting setup into SetupTrace() helper

- adding more debug info into SIGTRAP handling

- handling user-generated (and unknown) SIGTRAP events

- adding missing error handling to the generic signal handler

- fixing attaching to processes

- switching watchpoint helpers to use llvm::Error

- minor style and formatting changes

This fixes a number of tests, mostly related to fixed attaching.

Differential Revision: https://reviews.llvm.org/D91167